Passthrough adapters are mounted in holes of panels to provide passageways through which electric or optical cables can pass, or at which connections can be made. A common type of adapter includes a tube that is inserted into a hole in the panel, the tube having a flange which abuts the front face of the panel and having a thread. The thread receives a nut that presses a washer against the rear face of the panel. In many panel applications, there is very little room around the adapter for applying a wrench to the nut to tighten it. If an adapter could be installed without requiring a wrench or other tool to lie closely around it, especially if the adapter could be installed by merely pushing it into the hole until it snapped into place, this would provide a more easily used adapter. The adapter should be of low cost and able to securely mount in panels of a range of thicknesses.